Messages transmitted to cellular phones are encoded in order to reduce the chances that noise will corrupt the message. The message comprise a plurality of bits which are either ‘0’ or ‘1’. The encoding of the message usually includes adding redundancy (i.e., making the encoded message longer than the original message), involving symbol dependency (i.e., making the value of each symbol in the encoded message a function of a plurality of bits in the original message) and interleaving the message (i.e., mixing the order of the symbols in the encoded message).
Normally, each symbol in the received message is represented in the receiver, before decoding, by a word which represents the chance that the symbol is a logical “1”. A high positive value of the word means that the symbol is a logical ‘1’ with high confidence, while a negative value with a high absolute value means the symbol is a logical ‘0’ with high confidence. A word with a zero value means that the symbol is a ‘0’ or ‘1’ with equal probability. This representation is referred to herein as soft data.
The decoding of the message is usually performed by finding an original message which would result in the received encoded message with the highest probability. Due to the interleaving and the interdependence of the symbols of the encoded message the decoding process does not usually begin before the entire message, or an entire frame of a predetermined length, was received. If the message were not interleaved some decoding schemes would allow the decoding to begin before the entire message was received and end when the entire message is received. However, such schemes usually have a reduced performance, i.e., a higher rate of failures in correctly decoding the message, in particular in fading channels as often encountered in practice.
The messages usually include an error detection code which is used to determine whether the message was properly decoded. One such code is the cyclic redundancy code (CRC).
Most cellular phones operate on rechargeable batteries which provide power for their operation. Some cellular systems reduce the rate at which the cellular phones consume the battery power by having the phones operate in an idle mode when they are open for receiving calls but are not transmitting or receiving audio data. Generally, in the idle mode, the cellular phones deactivate most of their components to reduce their current consumption. Periodically, for example once every two seconds, the cellular phones activate all their components, for a short wake up period during the idle mode. A base station of the system sends a broadcast or paging message to the cellular phones on a paging channel, notifying them whether they are to remain in the idle mode or they are to change to a reception mode, for example in order to receive an incoming call.
The wake up period of a cellular phone comprises a warm-up period, a reception period, a decoding period and a shut down period. In the wake up period the cellular phone activates all of its components in order to receive the message. If the message tells the cellular phone to remain in the idle mode, the cellular phone shuts down most of its components during the shut down period. Any reduction in the length of the wake up period results in an increase in the time a cellular phone may be used without recharging or replacing its batter.